This invention concerns a temperature measuring device for enameled equipment.
In U.S. Pat. No. 3,830,105, there is disclosed a temperature measuring device of the four wire connecting type, having a tape-like platinum resistance wire having a cross section of approximately 0.003mm embedded in an enamel layer. This wire may be adjusted via a tuning link. The device is manufactured by placing the resistance wire on a first cover coat layer, and by coating and melting it into a second layer of cover coat. A window in the second layer is left over a measuring loop. Within the area of this window adjacent pieces of wire of the measuring loop may be bridged to form the adjusting link in accordance with a predetermined resistance of the measuring loop. After this procedure the window is covered with at least one additional layer of cover coat which is then melted. Such temperature measuring devices have proven to work well under operating conditions when using a four wire connection.
A two-line connection, on the other hand, has generally been found to be unsatisfactory, since the feed lines embedded along an enameled pipe or similar equipment cause an additional, temperature dependent resistance of approximately 10 ohms per meter, thereby preventing reliable temperature measurement. On the other hand the use of feed lines having a considerably larger diameter is generally not possible for reasons of enameling technology. Furthermore, the embedding of relatively long resistance wire and feed lines necessitates very careful work since the mechanical stress to which the lines are subjected is the greater the longer the lines are.
It is therefore the purpose of this invention to improve a temperature measuring device as described above, largely avoiding the described drawbacks and difficulties, so that the formation of a window and creation of a tuning link are not necessary. The resulting two-line connection has a measuring accuracy which is comparable with that achieved with the phantom connection.